At 2.5 seconds, the helicopter has a kinetic energy of 20 J. What is it mechanical energy at that time?

The mechanical energy of an object is equivalent to the sum of its potential and kinetic energy. If the helicopter is grounded, then its mechanical energy will be 20 J. If it is in the air, then its potential energy will be calculated using:

P. E = mgh

And then added to the kinetic energy to obtain mechanical energy.